x3::eps with a semantic action breaks parsing into structs

[syyyr]

Hi, I'm trying to get rid of fusion stuff in my single-element structs as suggested in #463. While doing that, I found a bug with semantic actions, where the struct doesn't get correctly filled with the value. This is how to reproduce.

#include <iostream>
#include <boost/spirit/home/x3.hpp>

namespace x3 = boost::spirit::x3;

struct number {
  int value;
  number& operator=(const int data) {
      this->value = data;
      return *this;
  }
};

auto doSideEffects = x3::eps[([] (auto& ctx) {
    // do some stuff...
})];

auto wrappedDoSideEffects = x3::rule<class Rule, x3::unused_type>{} = doSideEffects;

auto my_rule = x3::rule<class my_class, number>{} = doSideEffects > x3::int32;

int main(int argc, char* argv[])
{
    std::string input = "235";
    number out;
    auto it = input.begin();
    auto res = x3::parse(it, input.end(), my_rule, out);
    // This doesn't print 235, because doSideEffects breaks the parser.
    // If I swap doSideEffects with wrappedDoSideEffects, it works again.
    std::cout << "out.arg = " << out.value << "\n";

    return 0;
}

My operator= actually never gets called. I'm not sure I implemented it correctly. Is this really a bug or am I doing something wrong? I have also tried adding another member into the struct and use BOOST_FUSION_ADAPT_STRUCT and the bug is still there: https://gist.github.com/syyyr/94f7f9a0b65e8d4a81eb7a55be8e62ff

Boost version: 1.75.0

[djowel]

Semantic actions on a rule definition inhibit automatic attribute propagation. See:

https://stackoverflow.com/questions/33929849/boost-spirit-x3-ast-not-working-with-semantic-actions-when-using-separate-rule-d

[syyyr]

So, the function of operator%= is to automatically propagate attributes even if I'm using semantic actions?

[syyyr]

Okay, found it in the documentation of Qi. https://www.boost.org/doc/libs/1_75_0/libs/spirit/doc/html/spirit/qi/reference/nonterminal/rule.html